ARM: at91/clock: fix PLLA overclock warning

Fix PLLA overclock warning in relation with datasheet numbers.
Add new > 240 MHz and > 210 MHz SoC categories.

Reported-by: Jiri Prchal <jiri.prchal@aksignal.cz>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>